1. Field of Invention
The invention relates to a metallization structure over a semiconductor chip or wafer and the method for forming the same, and more particularly to a metallization structure over a copper pad and the method for forming the same.
2. Description of Related Arts
The reduction of the feature sizes of semiconductor devices using state-of-the-art semiconductor techniques have dramatically increased the device packing density of a single integrated circuit (IC) chip. However, as the device packing density increases, it is necessary to increase the number of electrical metal interconnect layers on the IC chip to effectively wire up the discrete devices on a substrate while reducing the chip size. For example, having two to six levels of metal interconnect layers in a single IC chip is a commonplace in this field.
After completing the multilevel interconnect structure, bonding pads are formed on the top surface of the interconnect structure to provide external electrical connections to the chip or die. A passivation layer is applied, such as silicon oxide, silicon nitride, silicon oxy-nitride or a combination thereof to protect the chip from moisture and contamination. After the passivation layer is formed, die containing a plurality of circuit patterns is connected to a package substrate. The package substrate may have a plurality of pins for connecting the circuitry to an external printed circuit board.
One method for forming electrical connections between the die and the package substrate is wire bonding. A corresponding set of contacts is located on the package substrate. A connecting wire is bonded to connect each bonding pad to a respective contact on the package substrate, using a method such as ultrasonic bonding. Following wire bonding, the package is encapsulated and sealed.
The reliability of the bonding process is particularly critical since the bonding process occurs so late in the production cycle. Die being packaged have typically already been tested and sorted. Any problems in the wire bonding process thus impact only good die. Secure, reliable bonding of the wire to the bonding pad requires that the bonding pad be formed of metals compatible with the bonding process. Aluminum and aluminum alloys are typically employed to achieve the most reliable bonds.
To prevent the shifting of bonding wires during the step of injecting the plastic material or the lengthening of the bonding wires, the bonding pads have been disposed on the peripheral of the chips. Therefore, longer conductive traces are needed to connect the device to the bonding pads. As the trend of chip advances toward higher speeds and higher capabilities, the number of I/O connections rapidly increases. However, the high inductance created in the connection of bonding pads and bonding wires obstructs the high-speed operation of the chips.